Allow executor startup pruning to prune all child nodes.

Previously, if the startup pruning logic proved that all child nodes
of an Append or MergeAppend could be pruned, we still kept one, just
to keep EXPLAIN from failing.  The previous commit removed the
ruleutils.c limitation that required this kluge, so drop it.  That
results in less-confusing EXPLAIN output, as per a complaint from
Yuzuko Hosoya.

David Rowley

Discussion: https://postgr.es/m/001001d4f44b$2a2cca50$7e865ef0$@lab.ntt.co.jp

src/include/nodes/execnodes.h

@@ -1234,8 +1234,6 @@ struct AppendState
  *		slots			current output tuple of each subplan
  *		heap			heap of active tuples
  *		initialized		true if we have fetched first tuple from each subplan
- *		noopscan		true if partition pruning proved that none of the
- *						mergeplans can contain a record to satisfy this query.
  *		prune_state		details required to allow partitions to be
  *						eliminated from the scan, or NULL if not possible.
  *		valid_subplans	for runtime pruning, valid mergeplans indexes to
@@ -1252,7 +1250,6 @@ typedef struct MergeAppendState
 	TupleTableSlot **ms_slots;	/* array of length ms_nplans */
 	struct binaryheap *ms_heap; /* binary heap of slot indices */
 	bool		ms_initialized; /* are subplans started? */
-	bool		ms_noopscan;
 	struct PartitionPruneState *ms_prune_state;
 	Bitmapset  *ms_valid_subplans;
 } MergeAppendState;